vue-vben-admin/src/layouts/default/header/LayoutMultipleHeader.less

.multiple-tab-header {
  flex: 0 0 auto;

  &.dark {
    margin-left: -1px;
  }

  &.fixed {
    position: fixed;
    top: 0;
    z-index: 100;
    width: 100%;
  }
}